Direct current system fault location method based on improved variable frequency signal injection method

Abstract

The application provides a DC system fault positioning method based on an improved variable frequency signal injection method, comprising the following steps: first, injecting AC signals with different frequencies into the bus of a DC system respectively, and collecting the response currents of each branch after the injection of the AC signals; then, screening the response currents through a signal processing algorithm to obtain response characteristic quantities, and calculating the response ratios of each branch according to the response characteristic quantities; finally, judging the branch with faults through a numerical comparison method based on the response ratios of each branch. The application injects variable frequency signals from both ends, combines branch feedback detection and corresponding signal algorithms, processes the feedback results by ratio, not only simplifies the operation steps and reduces the demand for detection equipment, but also reduces the influence of interference signals on the final judgment results through algorithm combination processing, thereby improving the overall detection accuracy of the method.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electric data processing, and in particular to a direct current system fault positioning method based on an improved variable frequency signal injection method. BACKGROUND

[0002] The direct current system of a substation provides power supply for control devices, relay protection devices and the like, and guarantees the normal operation of a power system. Due to the complex topology relationship of the direct current system and the various working conditions, component faults, secondary circuit faults and grounding faults often occur. Among them, the most dangerous is the grounding fault.

[0003] When a grounding fault occurs in the direct current system, the system can still work normally, but if the fault branch is not repaired in time, once another grounding fault occurs, a large short-circuit current will be generated to cause the fuse to open, the direct current system cannot supply power, the control devices, relay protection devices and the like cannot work, and further greater losses will occur. Therefore, when a grounding fault occurs in the direct current system, it needs to be repaired as soon as possible to avoid the simultaneous occurrence of two grounding faults.

[0004] The traditional grounding fault positioning methods include a signal injection method and a leakage current method. The leakage current method is not affected by the distributed capacitance of the direct current system relative to the ground, but when the insulation conditions of the positive and negative poles are similar, the leakage current method cannot detect the condition that the positive and negative poles are grounded at the same time. The signal injection method is easily affected by factors such as distributed capacitance, ring network harmonics and ripple voltage, so that the measured signal contains a large amount of interference, and the detection result has a large error.

[0005] The existence of the above problems leads to low detection accuracy of the existing grounding fault positioning method. Further, like the traveling wave method, more complex monitoring equipment is needed to accurately capture the traveling wave, the bridge method may have a large error for long distance branches, and the insulation monitoring device also has accuracy problems. Therefore, the existing technology also has problems such as high cost of detection equipment, low execution efficiency, and difficulty in quickly positioning the branch where the fault occurs when facing a complex circuit network. SUMMARY

[0006] In view of the problems existing in the prior art, the present application provides a substation direct current system branch fault positioning method based on an improved variable frequency signal injection method, which injects a variable frequency signal from both ends of a bus, combines branch detection and signal processing algorithms, and realizes the influence of factors such as distributed capacitance and ring network harmonics on the response current of the branch, thereby improving the detection accuracy of the variable frequency signal injection method.

[0007] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0008] The present application provides a direct current system fault positioning method based on an improved variable frequency signal injection method, which mainly includes the following steps:

[0009] injecting a first alternating current signal to a positive bus of a direct current system, and collecting a first response current of each branch;

[0010] after collecting the first response current of each branch, injecting a second alternating current signal to a negative bus of the direct current system, and collecting a second response current of each branch;

[0011] the frequency of the first alternating current signal is different from the frequency of the second alternating current signal;

[0012] decomposing the first response current and the second response current of each branch collected by a modal decomposition algorithm into a plurality of modal components, and screening out a modal component with a center frequency consistent with the frequency of the corresponding injected alternating current signal, and recording the modal component as a response modal component;

[0013] solving the amplitude of the response modal component based on a signal amplitude algorithm to obtain a response current amplitude;

[0014] solving the ratio of the amplitudes of the two response currents in each branch to obtain a response ratio of each branch;

[0015] judging the branch with a fault based on the response ratio of each branch by a numerical comparison method.

[0016] Optionally, the modal decomposition algorithm is a successive variational modal decomposition algorithm; and the signal amplitude algorithm is a Hilbert transform.

[0017] Optionally, the solving of the response modal component includes the following steps:

[0018] updating a Fourier transform formula of a current order modal in a Fourier domain to obtain a current order Fourier transform formula;

[0019] calculating and updating the center frequency of the current order modal according to the current order Fourier transform formula;

[0020] updating a Lagrange multiplier by a dual ascent method;

[0021] repeating the updating operation until an internal convergence condition is met, and outputting the current order modal component;

[0022] incrementing the modal order, continuing the updating operation and the convergence judgment until an external convergence condition is met, to complete the decomposition of the branch response current.

[0023] Optionally, the solving of the amplitude of the response modal component includes the following steps:

[0024] performing Hilbert transform processing on the response modal component to construct an analytic signal, and calculating the amplitude of the response modal component based on the analytic signal.

[0025] Optionally, the solving of the amplitude of the response modal component further comprises the following steps:

[0026] The Hilbert transform result is obtained by performing Hilbert transform on the response modal component through convolution operation.

[0027] The analytic signal is a signal function constructed by the response modal component and the Hilbert transform result.

[0028] The amplitude of the response modal component is calculated based on the signal function.

[0029] Optionally, the frequency of the first alternating current signal is greater than the frequency of the second alternating current signal.

[0030] Optionally, the solving of the response ratio comprises the following steps:

[0031] The response current amplitude corresponding to the first alternating current signal and the response current amplitude corresponding to the second alternating current signal in each branch are calculated.

[0032] The ratio of the two response current amplitudes in the branch is solved to obtain the response ratio of the branch.

[0033] Optionally, the response ratio is the ratio of the response current amplitude after the response current amplitude.

[0034] The branch with a fault comprises the following steps:

[0035] The branches are sorted according to the numerical value of the response ratio to obtain a branch sequence.

[0036] The branch with the largest response ratio or the response ratio greater than a threshold value in the branch sequence is selected as the branch with a fault.

[0037] Optionally, the method further comprises the following steps:

[0038] In the branch with a fault, a plurality of branch sections are divided based on branch nodes.

[0039] Different frequency branch alternating current signals are injected at both ends of each branch section.

[0040] The response currents of each branch section after the injection of the branch alternating current signals are sequentially collected, and the step of judging the branch with a fault is repeated to judge the branch section with a fault.

[0041] Optionally, the branch alternating current signal comprises a third alternating current signal and a fourth alternating current signal; and the response current after the injection of the branch alternating current signal comprises a third response current and a fourth response current.

[0042] injecting the third AC signal to the upstream node of the branch section, and collecting the third response current of the downstream node of the branch section;

[0043] after collecting the third response current, injecting the fourth AC signal to the downstream node of the branch section, and collecting the fourth response current of the upstream node of the branch section;

[0044] The frequency of the third AC signal is greater than the frequency of the fourth AC signal.

[0045] Compared with the prior art, the present application has the following beneficial effects:

[0046] The method of the present application injects variable frequency signals from both ends, combines branch feedback detection and corresponding signal algorithms, uses ratio processing of feedback results, simplifies operation steps on the one hand and reduces the demand for detection equipment; on the other hand, through algorithm combination, it filters out the interference parts such as harmonic and ripple voltage in the measured signal, alleviates the influence of distributed capacitance, and reduces the influence of interference signals on the final judgment result through variable frequency feedback signal ratio processing, thereby improving the overall detection accuracy of the method. BRIEF DESCRIPTION OF DRAWINGS

[0047] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor under the premise of the drawings.

[0048] Figure 1 The method flowchart of the embodiments of the present application;

[0049] Figure 2 The substation DC system model in the embodiments of the present application;

[0050] Figure 3 The normal branch model of the substation DC system in the embodiments of the present application;

[0051] Figure 4 The branch model of the substation DC system with ground fault in the embodiments of the present application. DETAILED DESCRIPTION

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0054] It is worth noting that, unless otherwise specified, the methods used in this invention are all conventional methods; and the raw materials and equipment used are all conventional commercially available products, and their sources are not specifically limited.

[0055] This embodiment provides a fault location method for DC systems based on an improved frequency conversion signal injection method, such as... Figure 1 As shown, it mainly includes the following steps:

[0056] When a fault is detected in the DC system of the substation, AC signals of different frequencies are first injected into the positive and negative buses of the DC system respectively. In this embodiment, the signal is injected once at both ends, and the AC signals of different frequencies include a first AC signal and a second AC signal.

[0057] Specifically, a signal generator is used to inject a first AC signal into the positive bus of the DC system. In this embodiment, a frequency of [frequency value missing] is selected. AC signal As the first AC signal, the first response current of each branch is measured using a current transformer. .

[0058] In the above examples, n The subscript represents the branch number, such as the superscript of the response current, and is counted. n A side road.

[0059] After collecting the first response current of each branch, a second AC signal is injected into the negative bus of the DC system, i.e. Figure 2 As shown, a signal generator is used to inject a frequency of [frequency value] only into the negative bus. AC signal As a second AC signal, the second response current of the branch is measured using a current transformer. .

[0060] The frequency of the first AC signal is greater than the frequency of the second AC signal. .

[0061] The response current is processed by a signal processing algorithm, which includes a mode decomposition algorithm. The acquired response current is decomposed into multiple mode components by the mode decomposition algorithm, and the mode component whose center frequency is consistent with the frequency of the injected AC signal is selected and denoted as the response mode component.

[0062] Specifically, the mode decomposition algorithm uses successive variational mode decomposition (SVMD) to decompose the current signal of each branch, including the following steps:

[0063] Initialize the iteration counter , indicating the number of modal components that have been extracted;

[0064] Repeat the following iterative process until the internal convergence condition is met, and output the current modal component. :

[0065] (a) For the current modal component Update its Fourier transform in the Fourier domain. The specific update formula is as follows:

[0066] ;

[0067] In the formula, For the first The first mode in the 1st order The updated Fourier transform; It is the Fourier transform of the input AC signal; It is a Lagrange multiplier Fourier transform; It is the first The first mode in the 1st order The center frequency after the next update; For balance parameters; It is a frequency variable in the Fourier domain. It is the first i The center frequency of the first modal component.

[0068] (b) Update the current order mode components center frequency The specific update method is as follows:

[0069] ;

[0070] In the formula, For the first The first mode in the 1st order The center frequency after the next update; For the first The first mode in the 1st order The Fourier transform after the next update.

[0071] (c) Update the Lagrange multipliers using the dual ascent method The specific update method is as follows:

[0072] ;

[0073] In the formula, It is the first Updated Lagrange multipliers Fourier transform; It is the dual ascending step size; It is the Fourier transform of the input signal; For the first The first mode in the 1st order Fourier transform after the next update; For the first Fourier transform of the first mode; It is the Fourier transform of the unprocessed signal.

[0074] Update iterative counter The modal order is increased incrementally. Specifically, the above steps are repeated to complete the update operation and convergence judgment, and modal components are extracted one by one until the external convergence condition is met, thereby obtaining the intrinsic mode functions (IMF) of each order. At this point, the branch response current of each branch is decomposed to obtain the set of intrinsic mode functions of each order [IMF1, IMF2, ..., IMFn].

[0075] The amplitude of the response mode components is determined based on a signal amplitude algorithm. In this embodiment, the signal amplitude algorithm used is the Hilbert transform. The steps for determining the amplitude of the response mode components are as follows:

[0076] The response modal components are subjected to Hilbert transform to construct an analytic signal, and calculations are performed based on the analytic signal; the specific steps are as follows:

[0077] The Hilbert transform results are obtained by performing a convolution operation on the response mode components.

[0078] An analytic signal is a signal function constructed from the response modal components and the Hilbert transform result;

[0079] The amplitude of the response modal components is calculated based on the signal function.

[0080] Therefore, in this embodiment, the response currents for the first and second AC signals are calculated respectively.

[0081] First, the Hilbert transform is used to calculate the injected signal in each branch. response current amplitude under the action of the injected signal to calculate the amplitude of the branch of the branch For example, specifically:

[0082] the response current of the branch of the branch In the foregoing step has been decomposed into modal components .

[0083] Select the center frequency of the modal component of the same order as the injected signal frequency , and perform Hilbert transform on it, the specific transform method is as follows:

[0084] ;

[0085] In the formula, is the Hilbert transform of the modal component of the order ; is the modal component of the order , and the center frequency is equal to the frequency of the injected alternating current signal ; denotes convolution operation.

[0086] Construct the analytic signal, the specific construction method is as follows:

[0087] ;

[0088] In the formula, is the analytic signal obtained by Hilbert transform; is the modal component of the order , and the center frequency is equal to the frequency of the injected alternating current signal ; is the Hilbert transform of the modal component of the order ; is the imaginary unit.

[0089] Thus, the amplitude function can be obtained as follows:

[0090] ;

[0091] In the formula, is the amplitude function of the modal component of the order , and is also the amplitude of the response current of the branch ​is the n-th modal component whose center frequency is equal to the frequency of the injected AC signal ; is the n-th modal component whose Hilbert transform is

[0092] Further, the response current amplitudes of each branch under the action of the injected signal are calculated by Hilbert transform to calculate the amplitude of the n-th branch For example, specifically:

[0093] The response current of the n-th branch is which has been decomposed into modal components in the foregoing steps ;

[0094] The n-th modal component whose center frequency is equal to the frequency of the injected signal is selected , and its Hilbert transform is , specifically as follows:

[0095] ;

[0096] wherein is the Hilbert transform of the n-th modal component ; is the n-th modal component whose center frequency is equal to the frequency of the injected AC signal ; represents convolution operation. The analytic signal is constructed, specifically as follows:

[0097]

[0098] ; wherein

[0099] is the analytic signal obtained by Hilbert transform; is the n-th modal component whose center frequency is equal to the frequency of the injected AC signal ; is the Hilbert transform of the n-th modal component ; is the imaginary unit.

[0100] ​​Thus, the amplitude function can be obtained as As follows:

[0101] ;

[0102] In the formula, is the amplitude function of the mth modal component, and is also the amplitude of the response current of the mth branch; is the mth modal component, and the center frequency is equal to the frequency of the injected alternating current signal; is the Hilbert transform of the mth modal component .

[0103] The ratio of the amplitudes of the two response currents in each branch is solved to obtain the response ratio of each branch. In the embodiment, the ratio discrimination method first calculates the amplitudes of the response currents corresponding to the alternating current signals of different frequencies in each branch, that is, the amplitudes of the first and second response currents in each branch, and compares the two response characteristics in the branch in time sequence to obtain the response ratio of the branch. The time sequence comparison method is to divide the response current amplitude obtained by subsequent acquisition and calculation by the response current amplitude obtained by previous acquisition and calculation.

[0104] Specifically, taking the mth branch as an example, the response ratio is .

[0105] Based on the response ratios of the branches, the numerical comparison method is used to determine the branch with a fault. The method of the embodiment is to sort the response ratios of the branches according to the numerical values of the amplitude ratios of the response currents, to obtain a branch sequence, and to select the branch with a response ratio greater than a threshold value or the largest response ratio in the branch sequence as the branch with a fault.

[0106] Specifically, the amplitude ratios of the response currents of the branches are compared, and the branch with a large amplitude ratio is the branch with a ground fault. The principle is as follows, assuming that there is no interference:

[0107] After the alternating current signal with the frequency is injected into the positive bus, the response currents of the branches are measured using the transformer. Assuming that the response current in the normal branch is , and the response current in the branch with a ground fault is ; after the alternating current signal with the frequency is injected into the negative bus, the response currents of the branches are measured using the transformer. Assuming that the response current in the normal branch is​​​​ , the response current in the branch with ground fault is ;

[0108] As Figure 3 described, the amplitude ratio of the response currents and in the normal branch is calculated as follows:

[0109] ;

[0110] In the formula, is the response current in the normal branch after injecting an AC signal with frequency from the positive bus; is the response current in the normal branch after injecting an AC signal with frequency from the negative bus; is the ground capacitance of the branch; is the voltage of the AC signal; is the frequency of the AC signal injected from the positive bus; is the frequency of the AC signal injected from the negative bus.

[0111] As Figure 4 shown, the amplitude ratio of the response currents and in the branch with ground fault is calculated as follows:

[0112] ;

[0113] In the formula, is the response current in the branch with ground fault after injecting an AC signal with frequency from the positive bus; is the response current in the branch with ground fault after injecting an AC signal with frequency from the negative bus; is the ground capacitance of the branch; is the voltage of the AC signal; is the ground resistance; is the frequency of the AC signal injected from the positive bus; is the frequency of the AC signal injected from the negative bus.

[0114] Due to , the following relationship shown in the formula can be obtained by the aforementioned formula, which indicates that the branch with larger amplitude ratio is the branch with ground fault:

[0115] ;

[0116] In the formula, is the frequency of the AC signal injected from the positive bus the response current in the normal branch after injecting the AC signal with the frequency of for the negative bus; the response current in the normal branch after injecting the AC signal with the frequency of for the positive bus; the response current in the branch with the ground fault after injecting the AC signal with the frequency of for the negative bus; the response current in the branch with the ground fault after injecting the AC signal with the frequency of

[0117] Therefore, it can be analyzed that, regardless of the load condition of the branch, when the ground fault occurs, the node position presents a resistance characteristic, thereby causing the ratio to abnormally increase, and a large number of interference factors can be excluded to obtain the condition that the branch has a fault.

[0118] Correspondingly, in order to simplify the judgment process, a more optimal way is to combine the threshold judgment method, collect the response ratio of each branch under normal working conditions multiple times, and take the maximum value of the multiple test results as the threshold of each branch. In subsequent measurement, the response ratio obtained above is compared with the threshold of the corresponding branch. When the response ratio is greater than the threshold, it means that the branch has a ground fault.

[0119] Optionally, in order to further quickly locate the fault position in the branch, the foregoing method can also be used for preliminary position judgment, and the specific steps are as follows:

[0120] The branch with the fault is judged through the foregoing steps, and multiple branch sections are divided based on the node of the branch. Thus, it is equivalent to a branch branch.

[0121] Different frequencies of branch AC signals are injected at both ends of each branch section; wherein, the branch AC signal includes a third AC signal and a fourth AC signal , and the frequency of the third AC signal is greater than the frequency of the fourth AC signal ; correspondingly, the response current after the branch AC signal includes a third response current and a fourth response current.

[0122] The injection mode is executed in sections in turn, and in each section, the third AC signal is injected to the upstream node of the branch section, and the third response current of the downstream node of the branch section is collected.

[0123] After the third response current is collected, the fourth AC signal is injected to the downstream node of the branch section, and the fourth response current of the upstream node of the branch section is collected.

[0124] The third and fourth response currents of each branch section after injecting the branch AC signal are collected in sequence, and the step of judging the branch with the fault is repeated, that is, the response ratios of each branch section are also calculated in the embodiment, and the response ratios of all sections in the branch with the fault are collected, and the section ratio sequence is obtained by sorting the ratios from large to small, wherein the largest ratio can be initially judged as the branch section with the fault, so as to perform on-site detection; when it is found that there is no ground fault after detection, the section ratio sequence is sequentially detected backward until the fault point is found.

[0125] Further, in the above implementation process of the section, the load on the load side of the branch needs to be excluded in the section rule, that is, the node before the load to the DC positive connection of the load is a section, and the DC negative connection of the load to the node after the load is a section.

[0126] In the above specific embodiment, the application injects a variable frequency AC signal at both ends, and judges the fault in the form of characteristic quantity ratio, which can effectively reduce the influence of interference, so that complex mathematical calculation steps or a large number of high-precision detection equipment are not required, the investment cost is reduced, the system response speed is improved, and the troubleshooting is fast, which reduces the occurrence of multi-point grounding fault; at the same time, the SVMD (successive variation mode decomposition) and Hilbert are used in the algorithm level to filter out the interference parts such as harmonic and ripple voltage in the measured signal, the influence of distributed capacitance is relieved, and the detection accuracy of the variable frequency injection method is further improved; further, the concept of equivalent branch is used to realize two-stage fault rapid positioning and judgment, which also saves the cost and reduces the work intensity of maintenance personnel.

[0127] Finally, it should be noted that the above content is only used to illustrate the technical solutions of the application, and is not a limitation on the protection scope of the application. Simple modifications or equivalent replacements of the technical solutions of the application made by those skilled in the art do not deviate from the essence and scope of the technical solutions of the application.

Claims

1. A DC system fault location method based on improved variable frequency signal injection method, characterized in that, The method comprises the following steps: injecting a first alternating current signal into a positive bus of a direct current system, and collecting a first response current of each branch; after collecting the first response current of each branch, injecting a second alternating current signal into a negative bus of the direct current system, and collecting a second response current of each branch; the frequency of the first alternating current signal is different from the frequency of the second alternating current signal; decomposing the collected first response current and the collected second response current of each branch into a plurality of modal components through a modal decomposition algorithm, and screening out a modal component with a center frequency consistent with the frequency of the injected corresponding alternating current signal, and denoted as a response modal component; solving the amplitude of the response modal component based on a signal amplitude algorithm to obtain a response current amplitude; solving the ratio of the two response current amplitudes in each branch to obtain a response ratio of each branch; judging the branch with a fault based on the response ratio of each branch through a numerical comparison method; the modal decomposition algorithm is a successive variational modal decomposition algorithm; and the signal amplitude algorithm is a Hilbert transform; the solving of the response modal component comprises the following steps: updating a Fourier transform formula of a current order modal in a Fourier domain to obtain a current order Fourier transform formula; the updating formula is as follows: ； In the formula, For the first The first mode in the first The Fourier transform after the next update; It is the Fourier transform of the input AC signal; It is a Lagrange multiplier Fourier transform; It is the first The first mode in the first The center frequency after the next update; For balance parameters; It is the frequency variable in the Fourier domain. It is the first i The center frequency of the first modal component; calculating and updating the center frequency of the current order modal according to the current order Fourier transform formula; updating a Lagrange multiplier through a dual ascent method; repeating the updating operation until an internal convergence condition is met, and outputting a current order modal component; incrementing the modal order, continuing the updating operation and the convergence judgment until an external convergence condition is met, so as to complete the decomposition of the branch response current; the solving of the amplitude of the response modal component comprises the following steps: performing Hilbert transform processing on the response modal component to construct an analytic signal, and calculating the amplitude of the response modal component based on the analytic signal; the response modal component is subjected to Hilbert transform through convolution operation to obtain a Hilbert transform result; the analytic signal is a signal function constructed by the response modal component and the Hilbert transform result; the amplitude of the response modal component is calculated based on the signal function.

2. The method for DC system fault location based on improved variable frequency signal injection method according to claim 1, characterized in that: the frequency of the first alternating current signal is greater than the frequency of the second alternating current signal.

3. The method for DC system fault location based on improved variable frequency signal injection method according to claim 2, characterized in that: the solving of the response ratio comprises the following steps: calculating the response current amplitude corresponding to the first alternating current signal and the response current amplitude corresponding to the second alternating current signal in each branch; solving the ratio of the two response current amplitudes in the branch to obtain the response ratio of the branch.

4. The method for DC system fault location based on improved variable frequency signal injection method according to claim 3, characterized in that: the response ratio is a later response current amplitude divided by an earlier response current amplitude in time sequence; the judgment of the branch with a fault comprises the following steps: sorting the branches according to the numerical value of the response ratio to obtain a branch sequence; selecting a branch with a response ratio greater than a threshold value or the largest response ratio in the branch sequence as the branch with a fault.

5. The method for DC system fault location based on improved variable frequency signal injection method according to claim 1, characterized in that: the method further comprises the following steps: dividing a plurality of branch sections based on branch nodes in the branch with a fault; injecting branch alternating current signals with different frequencies into both ends of each branch section; sequentially collecting the response current of each branch section after the injection of the branch alternating current signal, and repeating the step of judging the branch with a fault to judge a branch section with a fault.

6. The method for DC system fault location based on improved variable frequency signal injection method according to claim 5, characterized in that: The branch AC signals include a third AC signal and a fourth AC signal; the response currents after the branch AC signals include a third response current and a fourth response current; injecting the third AC signal to an upstream node of the branch section, and collecting a third response current of a downstream node of the branch section; after the third response current is collected, injecting the fourth AC signal to the downstream node of the branch section, and collecting a fourth response current of the upstream node of the branch section; the frequency of the third AC signal is greater than the frequency of the fourth AC signal.

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